1. Field of the Invention
The invention relates to gas turbine engine frames used to support bearings that support turbine rotors and, particularly, for static frames including inter-turbine frames and rotatable frames that are used in supporting counter-rotating low pressure turbine rotors.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some low pressure turbines have been designed with counter-rotating turbines that power counter-rotating fans and booster or low pressure compressors. U.S. Pat. Nos. 4,860,537, 4,758,129, 4,951,461, 5,307,622, 5,433,590 and 4,790,133 disclose counter-rotating turbines that power counter-rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan.
Engine frames including fan and turbine frames are used to support and carry the bearings which, in turn, rotatably support the rotors. Frames generally include struts, including struts with airfoil-shaped cross-sections, radially extending and mounted between co-annular radially inner and outer rings. Static aft turbine frames and inter-turbine frames are located at the aft end of the low pressure turbine and between high and low pressure turbines, respectively. Examples of static inter-turbine frames are disclosed in U.S. Pat. Nos. 4,976,102 and 5,483,792. The struts are generally cast because of the complexity of the flowpath through which the struts pass and because cast construction reduces manufacturing costs. Flowpath temperature is another reason for using cast alloy struts. Large modern commercial turbofan engines have higher operating efficiencies with higher bypass ratio configurations, larger transition ducts between low pressure and high pressure turbines. The frames, especially those located in the engine hot section, are complex and expensive.
Rotatable turbine frames are used in engine designs incorporating counter-rotating turbines. Examples of rotatable turbine frames are disclosed in U.S. Pat. Nos. 5,307,622, and 5,433,590. New commercial engine designs are incorporating counter-rotating rotors for improved turbine efficiency. The inner and outer rings, which carry the rotational loads, are generally made of a forged material due to forging materials exhibiting superior strength and fatigue characteristics. A need exists for engine frames, particularly in the hot turbine sections that will reduce engine length, weight, and cost.
Rotating frames will be subject to FAA LCF life limitation requirements, meaning that life to crack initiation must be calculated and the parts retired in revenue service at some fraction of that life. Alternatively, life may be established based on some probability of inherent defect propagation in the parts, depending on the material. The low fatigue properties of castings and the high level of defects inherent to the casting process make the design of a cast rotating frame with adequate service life difficult.
It is highly desirable to have a high temperature rotating frame construction that uses castings for the strut-airfoils and flowpath where the redundant nature of the components will allow them to be treated, for purposes of FAA certification, as turbine airfoils are now treated with respect to replacement for cause based on condition. It is highly desirable to use forging to construct the inner and outer load carrying rings. This will allow the rings to be treated as traditional rotating parts using traditional materials and manufacturing methods. It is also highly desirable for the rotatable frame to have a dynamically stiff structure with adequate strength, all while being mechanically simple for reasons of manufacturing cost and serviceability.